This invention relates to a motor operated valve of a type wherein a rotor provided in a tubular member is rotated by a stator provided outside of the tubular member, and a valve member is operated by the rotation of the rotor.
The known valves are disclosed, for instance, in published U.S. Pat. Nos. 4,452,423 and 4,789,132. The disclosed valve comprises a stator which generates a rotating magnetic field around the wall of a tubular member, and a rotor which is supported rotatably in the tubular member in opposition to the stator and a screw and a nut. The rotating movement of the rotor is converted into a linear movement of a valve member by a screw mechanism. The aforementioned stator is constructed in the form of a magnetic collar having permanent magnets and freely slidable around the tubular member, or in the form including ordinary stator coils. On the other hand, the rotor is constructed to include permanent magnets or a magnetic member adapted to create eddy current.
In a case where the stator is constructed in the form of a magnetic collar, a mechanism for slidably rotating the collar around the tubular member is required, while in a case where the stator is constructed to include ordinary stator coils, the outer measurement thereof will be increased when the magnetomotive force of the coils is desired to be increased. Furthermore, the number of magnetic poles is restricted by the shape and size of the coils, so that a precision adjustment of the valve member is made difficult. In addition, when the permanent magnets are used in the rotor, the magnetic property of the magnets tends to be deteriorated by the high temperature of the fluid, or a joining strength between the magnets and the supporting member, which strength also tends to be weakened similarly so that the magnets will be easily dropped out of the rotor.